AP Chemistry Electrochemistry Packet Unit 2

1

AP Chemistry Electrochemistry Packet Unit 2

Redox Review

Oxidation numbers are very important in this chapter “Redox Reactions.” Without the complete understanding of how to assign these numbers, we cannot move ahead with this chapter. They are much like ionic charges, except the every element will be assigned a number. The most important rules that cannot be broken are:

Free elements are zero. Group 1 is +1

Group 2 is +2 Fluorine is -1

Assign oxidation numbers to each element in the following: (Complete 9 from each side: MUST INCLUDE e, f, l, u, x, aa, bb, hh, xx)

(a) NaCl Na___ Cl___

(b) H2S H___ S___

(c) H2O H___ O ___

(d) CO2 C ___ O___

(e) H2SO4 H ___ S___ O___

(f) FeCO3 Fe___ C___ O___

(g) AgI Ag___ I___

(h) H2 H___

(i) PbCl2 Pb___ Cl___

(j) BaCO3 Ba___ C ___ O___

(k) Fe2O3 Fe___ O___

(l) I2 I____

(m) BeO Be____ O____

(n) CaF2 Ca____ F ____

(o) FeCl3 Fe____ Cl____

(p) PF5 P____ F____

(q) H3PO4 H____ P____ O___

(r) KCl K ____ Cl____

(s) K2O K ____ O____

(t) O3 O ____

(u) LiH Li ____ H____

(v) HBr H ____ Br____

(w) Li+ Li____

(x) PO43- P____ O___

(y) CaH2 Ca___ H___

(z) Cr2O3 Cr___ O___

(aa) KClO K___ Cl___ O___

(bb) KClO2 K___ Cl___ O___

(cc) KClO3 K___ Cl___ O___

(dd) KClO4 K___ Cl___ O___

(ee) Na2SO4 Na___ S___ O___

(ff) Ca(OH)2 Ca___ O___ H___

(gg) Na2SO4 Na___ S___ O___

(hh) B2(Cr2O7)3 B___ Cr___ O___

(ii) Al2(SO4)3 Al___ S___ O___

(jj) Al(NO3)3 Al___ N___ O___

(kk) (NH4)3PO4 N___ H___ P___ O____

2

Students should be able to demonstrate an understanding of the following essential knowledge: 3.B.3 In oxidation-reduction (redox) reactions, there is a net transfer of electrons. The species that loses electrons

is oxidized, and the species that gains electrons is reduced. 3.C.3 Electrochemistry shows the interconversion between chemical and electrical energy in galvanic and

electrolytic cells.

AP Electrochemistry Packet Unit 10Redox Reactions

A redox reaction is a reaction in which electrons are transferred from one element to another. The reaction involves at least two elements, one that will give up an electron, and one that will receive that electron. The term redox comes from two words, “oxidation” and “reduction.” If something is oxidized, it “burns” in oxygen, as shown below:

Mg + O Mg+2 + O-2

Take a moment to write out their Lewis dot structures. As you can see, Mg is being oxidized and it loses its two valence electrons, while oxygen gains them. Oxidation is defined as the loss of electrons.

Considering the name of this reaction, if something is being oxidized, the other element must be reducing. Neither reduction nor oxidation can happen alone. That means that oxygen, in this example, is being reduced because it gains the electrons. Reduction is defined as the gain of electrons.

A simple way to remember this is to remember that LEO the lion goes GER. LEO = loss of electrons is oxidation. GER = gain of electrons is reduction.

Also, Mg and O are “free” elements, which means their oxidation numbers are both 0. Now you can see that if and element’s oxidation number increases, that element is being oxidized. If an element’s oxidation number decreases it is being reduced (the number is reducing).

_____________________________________________________________________________

EXAMPLES: Indicate which element is being oxidized, and which is being reduced.

1. Cr3+ + Fe2+ Cr2+ + Fe3+

2. F2 + O2- F1- + O2

3. Sn + N5+ Sn4+ + N4+

4. NaCl Na+ + Cl-

5. Cu2O Cu + O2

6. Cl2 + KBr KCl + Br2

7. CH4 + O2 CO2 + H2O

8. H3PO4 + Ca(OH) 2 Ca3(PO4) 2 + H2O

Redox reactions are usually synthesis reactions, decomposition, combustion or single replacement reactions. Double replacement and neutralization reactions are NOT redox reactions. Usually they are easy to spot because if an element goes from being “free” (with an oxidation number of 0) to being in a compound (with a new oxidation number) it shows there was an exchange of electrons. In the following examples, identify what type of reaction they are and then state if they are redox reactions.

1. N2 + O2 2NO _________________________ ____________3

AP Electrochemistry Packet Unit 102. Cl2 + 2NaBr NaCl + Br2 _________________________ ____________

3. 2NaOH + HCl H2O + NaCl _________________________ ____________

Are these redox?

What type of reaction (S, D, C, SR, or DR) is NEVER redox? ______________________________

4

AP Electrochemistry Packet Unit 10Balancing Redox Reactions

Balance in an acidic solution:1. Cr2O7

2- + I- Cr3+ + IO3-

 

2. MnO4- + CH3OH Mn2+ + HCO2H

  3. I

2 + OCl- IO3- + Cl-

4. As2O3 + NO3- H3AsO4 + N2O3

Balance in a basic solution:

5. MnO4- + Br- MnO2 + BrO3

–

6. Pb(OH)42- + ClO- PbO2 + Cl-

 

7. Al + MnO4 - MnO2 + Al(OH)4

–

 

8. Cl2 Cl- + OCl–

 

9. NO2- + Al NH3 + AlO2-

5

AP Electrochemistry Packet Unit 10Electrochemical Cells

Directions: In each of the following, determine which element oxidized easier on table J. Then label the anode, cathode, direction of e- flow, and the half reactions. Then find the voltage.

1. 2.

3. 4.

Additional Questions:5. On diagram 1, which way will anions travel through the salt bridge? ____________6. On diagram 2, towards which electrode will cations travel through the salt bridge? ____________7. On diagram 3, how many e- are exchanged per mole of Mg? ____________8. On diagram 4, how many e- are transferred between Ag and Ni? ____________9. On all diagrams, at which electrode does oxidation occur? ____________10. On all diagrams, at which electrode does reduction occur? ____________11. On all diagrams, from which electrode will electrons travel? ____________12. What is the purpose of the salt bridge? ____________________________________________________13. Describe the change in energy that occurs in voltaic cells in terms of electric and chemical energies:1.

Students should be able to demonstrate an understanding of the following essential knowledge: 3.B.3 In oxidation-reduction (redox) reactions, there is a net transfer of electrons. The species that loses electrons

is oxidized, and the species that gains electrons is reduced. 3.C.3 Electrochemistry shows the interconversion between chemical and electrical energy in galvanic and

electrolytic cells.

6

AP Electrochemistry Packet Unit 10Redox Titrations

Like acids and bases, redox reactions can represent titrated substances. When an oxidizing agent or reducing agent is added to a solution the oxidation numbers what and often color changes can be seen. For example, when copper changes from Cu+2 to Cu+3 the color changes from blue to colorless. A list of color changes are below. Either dimensional anaylysis or titration eqiations can be used to determine the conenctration of species in the reaction.

1. (a) When 300.0 milliliters of a solution of 0.200 molar AgNO3 is mixed with 100.0 milliliters of a 0.0500 molar CaCl2 solution, what is the concentration of silver ion after the reaction has gone to completion?

(b) Write the net cell reaction for a cell formed by placing a silver electrode in the solution remaining from the reaction above and connecting it to a standard hydrogen electrode.

(c)Calculate the voltage of a cell of this type in which the concentration of silver ion is 410-2 M.

(d) Calculate the value of the standard free energy change G for the following half reaction:Ag+ (1M) + e- Ag

2. Answer parts (a) through (e) below, which relate to reactions involving silver ion, Ag+.The reaction between silver ion and solid zinc is represented by the following equation.

2 Ag+(aq) + Zn(s) Zn2+(aq) + 2 Ag(s)

(a)A 1.50 g sample of Zn is combined with 250. mL of 0.110 M AgNO3 at 25˚C.(i) Identify the limiting reactant. Show calculations to support your answer.

(ii) On the basis of the limiting reactant that you identified in part (i), determine the value of [Zn2+] after the reaction is complete. Assume that volume change is negligible.

Ti+3 and V+2 are purpleV+3 Cr+3, Fe+2, Co+3, and Ni+2 are bright, dark, aqua, olive, and blue-greenV+4, Cr+2, Cu+2 are bright, dark, and navy blueV+5 and Fe+3 are bright and light yellowMn+7 is magentaMn+2 and Co+2 are light and dark pinkCr+6 is orange

7

AP Electrochemistry Packet Unit 10

(b) Determine the value of the standard potential, E˚, for a galvanic cell based on the reaction between AgNO3(aq) and solid Zn at 25˚C.

Electrolysis Review and New

Students should be able to demonstrate an understanding of the following essential knowledge: 3.A.2 Quantitative information can be derived from stoichiometric calculations that utilize the mole ratios from the

balanced chemical equations. The role of stoichiometry in real-world applications is important to note, so that it does not seem to be simply an exercise done by only chemists.

3.B.3 In oxidation-reduction (redox) reactions, there is a net transfer of electrons. The species that loses electrons is oxidized, and the species that gains electrons is reduced.

3.C.3 Electrochemistry shows the interconversion between chemical and electrical energy in galvanic and electrolytic cells.

1. Which element is oxidizing in the diagram to the left?2. According to E Tables, should that element oxidize? 3. Is this reaction spontaneous?4. What type of cell is it?5. Label the anode and the cathode with charges on the diagram to the left.6. Write the overall reaction for this cell.

7. Water is being decomposed using a battery in the diagram to the right. Write the equation for the decomposition of water.

8. Which element is being oxidized? 9. How many e- are lost? 10. Should that element oxidize? 11. Is this reaction spontaneous?12. What type of cell is it?12. Label where oxidation and reduction on the diagram.13. Why is more H2 gas being formed that O2 gas?

14. Write the overall reaction for this cell.

8

AP Electrochemistry Packet Unit 10

AP Electrolysis

1. All of the equations on you reference table are written as (oxidations/reductions).

2. The chemicals at the upper left (Cl2 and O2) are the most likely to be (oxidized/reduced) and therefore the best (oxidizing agents/reducing agents).

3. The chemicals at the lower right (Na and K) are the most likely to be (oxidized/reduced) and therefore the best (oxidizing agents/reducing agents).

4. In an electrolytic cell, the () electrode is negative because it has (too many/too few) electrons. Chemicals that come into contact with the () electrode will (gain/lose) electrons and be (oxidized/reduced). The () electrode in electrolysis is called the (cathode/anode).

5. Write the change that water goes through at the () electrode.

6. In an electrochemical cell, the (+) electrode is positive because it has (too many/too few) electrons. Chemicals that come into contact with the (+) electrode will (gain/lose) electrons and be (oxidized/reduced). The (+) electrode in electrolysis is called the (cathode/anode).

7. Write the change that water goes through at the () electrode.

8. Add these two reactions together and write the overall reaction for the electrolysis of water.

9. We will perform this electrolysis using an aqueous solution of sodium sulfate.Both the Na+ and H2O will be near the () electrode. Which chemical is more likely to be reduced?

10. Both the SO42 and H2O will be near the (+) electrode. Which chemical will be oxidized?

15. Only one element is being used in the diagram to the left. The silver plate ionizes and the ions attach to the spoon. Show a half reaction for silver oxidizing.

16. Label the anode and cathode with charges on the diagram.17. Explain the direction of e- flow through the wire.

18. Is this reaction spontaneous? _____ How can you tell?

9

AP Electrochemistry Packet Unit 10

11. In the electrolysis of KI(aq)Both the K+ and H2O will be near the () electrode. Which chemical is more likely to be reduced? Both the I and H2O will be near the (+) electrode. Which chemical is more likely to be oxidized?Write the reactions at each electrode and the overall reaction:

Cathode:

Anode:

Overall:

12. In the electrolysis of CuSO4(aq)Both the Cu2+ and H2O will be near the () electrode. Which chemical will be reduced? Both the SO4

2 and H2O will be near the (+) electrode. Which chemical will be oxidized? Write the reactions at each electrode and the overall reaction:

Cathode:

Anode:

Overall:

13. Silver plating occurs when electrolysis of a Ag2SO4 solution is used because silver metal is formed at the (cathode/anode). This is the ( + / )electrode. The reaction at this electrode is:

Recall that 1 amp·sec = 1 Coulomb and 96,500 Coulombs = 1 mole e‘s (Faraday’s constant).If a cell is run for 200. seconds with a current of 0.250 amps, how many grams of Ag will be deposited?

14. A current of 10.0 amperes flows for 2.00 hours through an electrolytic cell containing a molten salt of metal X. This results in the decomposition of 0.250 mole of metal X at the cathode. The oxidation state of X in the molten salt is (X+, X2+, X3+, X4+)

15. Solutions of Ag+, Cu2+, Fe3+ and Ti4+ are electrolyzed with a constant current until 0.10 mol of metal is deposited. Which will require the greatest length of time?

10

AP Electrochemistry Packet Unit 1016. Based on the activity series what is the outcome of each of the following reactions?

a. Al (s) + NiCl2 (aq)

b. Pb(NO3) 2 (aq) + Ag (s)

c. Cr (s) + NiSO4 (aq)

d. Mn (s) + HBr (aq)

e. H2 (g) + CuCl2 (aq)

17. Using the activity series write the balanced chemical equations for the following reactions.

f. Zinc metal is added to a solution of silver nitrate.

g. Iron metal is added to a solution of aluminum sulfate.

h. Hydrochloric acid is added to cobalt metal.

i. Hydrogen gas is bubbled through an aqueous solution of FeCl2.

j. Lithium metal is added to water.AP Questions (Answer 6 of the following)1. Ti3+ + HOBr TiO2+ + Br- (in acid solution)

(a)Write the correctly balanced half-reactions and net ionic equation for the skeletal equation shown above.

(b) Identify the oxidizing agent and the reducing agent in this reaction.

(c)A galvanic cell is constructed that utilizes the reaction above. The concentration of each species is 0.10 molar. Compare the cell voltage that will be observed with the standard cell potential. Explain your reasoning.

(d) Give one example of a property of this reaction, other than the cell voltage, that can be calculated from the standard cell potential, E. State the relationship between E and the property you have specified.

11

AP Electrochemistry Packet Unit 10

2. (a) Titanium can be reduced in an acid solution from TiO2+ to Ti3+ with zinc metal. Write a balanced equation for the reaction of TiO2+ with zinc in acid solution.

(b) What mass of zinc metal is required for the reduction of a 50.00 millilitre sample of a 0.115 molar solution of TiO2+?

(c)Alternatively, the reduction of TiO2+ to Ti3+ can be carried out electrochemically. What is the minimum time, in seconds, required to reduce another 50.000 millilitre sample of the 0.115 molar TiO 2+ solution with a direct current of 1.06 amperes?

(e)The standard reduction potential, E, for TiO2+ to Ti3+ is +0.060 volt. The standard reduction potential, E, for Zn2+ to Zn(s) is -0.763 volt. Calculate the standard cell potential, E, and the standard free energy change, G, for the reaction described in part (a).

3. A direct current of 0.125 ampere was passed through 200 millilitres of a 0.25 molar solution of Fe2(SO4)3 between platinum electrodes for a period of 1.100 hours. Oxygen gas was produced at the anode. The only change at the cathode was a slight change in the color of the solution.At the end of the electrolysis, the electrolyte was acidified with sulfuric acid and was titrated with an aqueous solution of potassium permanganate. The volume of the KMnO4 solution required to reach the end point was 24.65 millilitres.

(a)How many faradays were passed through the solution?

(b)Write a balanced half-reaction for the process that occurred at the cathode during the electrolysis.

(c)Write a balanced net ionic equation for the reaction that occurred during the titration with potassium permanganate.

12

AP Electrochemistry Packet Unit 10

(d) Calculate the molarity of the KMnO4 solution.

4. An electrochemical cell consists of a tin electrode in an acidic solution of 1.00 molar Sn2+ connected by a salt bridge to a second compartment with a silver electrode in an acidic solution of 1.00 molar Ag+.

(a) Write the equation for the half–cell reaction occurring at each electrode. Indicate which half–reaction occurs at the anode.

(b) Write the balanced chemical equation for the overall spontaneous cell reaction that occurs when the circuit is complete. Calculate the standard voltage, E, for this cell reaction.

5. Explain each of the following.(a) When an aqueous solution of NaCl is electrolyzed, Cl2(g) is produced at the anode, but no Na(s) is produced at the cathode.

(b) The mass of Fe(s) produced when 1 faraday is used to reduce a solution of FeSO4 is 1.5 times the mass of Fe(s) produced when 1 faraday is used to reduce a solution of FeCl3.

(c) Zn + Pb2+ (1–molar) Zn2+ (1–molar) + PbThe cell that utilizes the reaction above has a higher potential when [Zn2+] is decreased and [Pb2+] is held constant, but a lower potential when [Pb2+] is decreased and [Zn2+] is held constant.

(d)The cell that utilizes the reaction given in (c) has the same cell potential as another cell in which [Zn 2+] and [Pb2+] are each 0.1–molar.

6. An unknown metal M forms a soluble compound, M(NO3)2.(a) A solution of M(NO3)2 is electrolyzed. When a constant current of 2.50 amperes is applied for 35.0 minutes, 3.06 grams of the metal M is deposited. Calculate the molar mass of M and identify the metal.

(b) The metal identified in (a) is used with zinc to construct a galvanic cell, as shown below. Write the net ionic equation for the cell reaction and calculate the cell potential, E.

13

AP Electrochemistry Packet Unit 10

K +

C l -

Z i n c

1 . 0 M Z n S O 4

M e t a l M

1 . 0 M M ( N O ) 3 2

(c) Calculate the value of the standard free energy change, G, at 25C for the reaction in (b).

(d) Calculate the potential, E, for the cell shown in (b) if the initial concentration of ZnSO4 is 0.10-molar, but the concentration of the M(NO3)2 solution remains unchanged.

8. In an electrolytic cell, a current of 0.250 ampere is passed through a solution of a chloride of iron, producing Fe(s) and Cl2(g).

(a) Write the equation for the half-reaction that occurs at the anode.

(b) When the cell operates for 2.00 hours, 0.521 gram of iron is deposited at one electrode. Determine the formula of the chloride of iron in the original solution.

(c) Write the balanced equation for the overall reaction that occurs in the cell.

(a)How many liters of Cl2(g), measured at 25C and 750 mm Hg, are produced when the cell operates as described in part (b) ?

(e)Calculate the current that would produce chlorine gas from the solution at a rate of 3.00 grams per hour.

9. 14

AP Electrochemistry Packet Unit 10Answer the following questions regarding the electrochemical cell shown.

(a)Write the balanced net-ionic equation for the spontaneous reaction that occurs as the cell operates, and determine the cell voltage.

(b) In which direction do anions flow in the salt bridge as the cell operates? Justify your answer.

(c)If 10.0 mL of 3.0-molar AgNO3 solution is added to the half-cell on the right, what will happen to the cell voltage? Explain.

(d) If 1.0 gram of solid NaCl is added to each half-cell, what will happen to the cell voltage? Explain.

(e)If 20.0 mL of distilled water is added to both half-cells, the cell voltage decreases. Explain.

10. Answer the following questions that refer to the galvanic cell shown in the diagram above. (a)Identify the anode of the cell and write the half reaction that occurs there.

(b)Write the net ionic equation for the overall reaction that occurs as the cell operates and calculate the value of the standard cell potential, Ecell .

15

AP Electrochemistry Packet Unit 10

(c)Indicate how the value of Ecell would be affected if the concentration of Ni(NO3)2(aq) was changed from 1.0 M to 0.10 M and the concentration of Zn(NO3)2(aq) remained at 1.0 M. Justify your answer.

11. The compound NaI dissolves in pure water according to the equation NaI(s) Na+(aq) + I–(aq). Some of the information in the table of standard reduction potentials given below may be useful in answering the questions that follow.

Half-reaction E˚ (V)O2(g) + 4 H+ + 4 e- 2 H2O(l) 1.23

I2(s) + 2 e- 2 I– 0.53

2 H2O(l) + 2 e- H2(g) + 2 OH– -0.83

Na+ + e- Na(s) -2.71(d) An electric current is applied to a 1.0 M NaI solution.

(i) Write the balanced oxidation half reaction for the reaction that takes place.

(ii) Write the balanced reduction half-reaction for the reaction that takes place.

(iii) Which reaction takes place at the anode, the oxidation reaction or the reduction reaction?

(iv) All electrolysis reactions have the same sign for ∆G˚. Is the sign positive or negative? Justify your answer.

12. 2 H2(g) + O2(g) 2 H2O(l)

In a hydrogen-oxygen fuel cell, energy is produced by the overall reaction represented above.

(a) When the fuel cell operates at 25˚C and 1.00 atm for 78.0 minutes, 0.0746 mol of O2(g) is consumed. Calculate the volume of H2(g) consumed during the same time period. Express your answer in liters measured at 25˚C and 1.00 atm.

16

AP Electrochemistry Packet Unit 10

(b) Given that the fuel cell reaction takes place in an acidic medium,(i) write the two half reactions that occur as the cell operates,

(ii) identify the half reaction that takes place at the cathode, and

(iii) determine the value of the standard potential, E˚, of the cell.

(c) Calculate the charge, in coulombs, that passes through the cell during the 78.0 minutes of operation as described in part (a).

17